ZF JSON is for preparing and parsing JSON style data (which is just an alternative to XML).

But as far as AJAX goes, the code you use depends on the JS code... are you using a library (jquery, prototype.js, mootools) or do you have custom code to do your AJAX calls?

That's the first thing to figure out.

Also, how is reporting the error? JS or ZF? If its JS, then look there. If its ZF, then more than likely the code your ajax is trying to call has a bug.